Lockheed Martin reveals F-35 to feature nanocomposite structures

Lockheed Martin has revealed the F-35 Lightning II will be the first mass-produced aircraft to integrate structural nanocomposites in non-load bearing airframe components.

A thermoset epoxy reinforced by carbon nanotubes will replace carbon fibre as the material used to produce F-35 wingtip fairings beginning with low rate initial production (LRIP)-4 aircraft, said Travis Earles, a manager for corporate nanotechnology initiatives.

Meanwhile, the same carbon nanotube reinforced polymer (CNRP) material is being considered to replace about 100 components made with other composites or metals throughout the F-35's airframe, he said.

The shift to CNRP as an airframe material has been anticipated ever since carbon nanotubes were discovered in 1991. It is widely considered one of the strongest materials ever invented - several times stronger than carbon fibre reinforced plastic (CFRP), yet lighter by about 25-30%.

CFRP has come to rival aluminium and steel as a material for primary structures: for instance, Boeing is building 787 fuselage barrels entirely from CFRP material.

But the widespread usage of CFRP for load-bearing components of commercial and military airframes only happened after two decades of development work.

That development cycle began in the early 1970s, when manufacturers began experimenting with glass and carbon fibre-based composites for secondary control surfaces.

Similarly, the introduction of CNRP in non-load bearing structures in the current decade, starting with the F-35, could lead to wider usage in airframe structures as the technology matures.

According to Earles, there is no technical reason the material could not be used in load-bearing structures. However, to reduce certification requirements of a structural material, carbon nanotubes are only being considered in non-load bearing components.

In the meantime, the use of carbon nanotubes is already widespread in industries ranging from semiconductors to golf clubs. The high cost and complexity of producing the structures means they have so far had limited applications in aerospace programmes.

Lockheed, however, has invented a process that dramatically reduces the cost to build carbon nanotube composites for aircraft structures, Earles said. The new wingtip fairing is being made for one-tenth of the cost of the equivalent CFRP component, he said.

Earles declined to describe details of Lockheed's low-cost manufacturing process for carbon nanotube composites. Such particulars are considered trade secrets within the company, he said.

But it is clear that the cost-saving process is a relatively new invention within Lockheed. Earles said it has evolved within the past four years. During the same time period, Lockheed has been active in developing new, low-cost methods for manufacturing airframe structures.

Lockheed was selected by the Air Force Research Laboratory in 2007 to build and demonstrate the X-55 advanced composite cargo aircraft (ACCA), which modified a Fairchild Dornier 328Jet. The fuselage was rebuilt with a new kind of carbon fibre resin that can be cured outside an autoclave, avoiding one of the most costly steps in the production of CFRP materials.

It is possible that the X-55 airframe also benefited from the development work that produced the carbon nanotube composites, which are now being applied to the F-35.

The company is only now publicising the first details about the rapid progress in manufacturing nanostructures for airframes.

The material is identified as "advanced polymers engineered for the extreme - first generation", or APEX. It is described within the display as "best-in-class ultra-lightweight and affordable structural thermoplastic enhanced with nanoparticles that delivers increased mechanical properties, thermal stability, electrical conductivity and processability over currently available projects".

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